The region is underlain by the Coast Plutonic Complex, consist- ing of pre- to post-kinematic quartz diorite to granodiorite batho- liths intruded into Paleozoic to Mesozoic metasedimentary and meta- volcanic rocks. Younger supracrustal assemblages overlying deformed rocks include the (?)Cretaceous Gambier Group and Tertiary Bella Bella Formation volcanic rocks.

The bedrock geology of the Tallheo hot springs comprises foli- ated granodiorite and schist of possible volcanic protolith. The waters of the hot spring are reported to be about 60 degrees celsius; no information on composition is available.